The present invention relates to the use of the Stark effect for acoustically changing the optical properties of a material.
In a solid material, the propagation of light is controlled by the electronic polarisation of the medium. For the simple case of a transparent medium, electronic excitation of the medium can be used to modify the light dispersion properties of the medium by modifying the refractive index. This produces a constant change in the speed of light through the medium. In this way a coherent beam or pulse of acoustic phonons in a material will modify the properties of the material in order to change the refractive index of that material.
When there are electronic resonances at particular energies, the dielectric function has a resonance at the energy of the electronic excitation, and strong frequency-dependence. The excitations may come about from, from example, localised electronic excitations as in impurity levels in a glass, or extended electronic excitations at a semiconductor band edge, or excitations of bound electron-hole pairs (excitons) in a semiconductor.
Because of the electronic resonance, the optical field and the polarisation are coupled, and new coupled modes are generically termed polaritons. A polariton spectrum is shown in FIG. 1.
The Stark effect refers to the modulation of the frequency of the electronic resonance by the application of a field, which may typically be an electrical field or an optical field.
In the electrical Stark effect, a d.c. or low frequency (GHz) voltage bias is applied to shift the position of the resonance, and one common application of this is as a laser modulator—the band gap of a semiconductor is lowered so as to cause absorption of the incident laser light, and thereby to produce extinction of the light propagation.
The optical Stark effect mediated by an excitonic resonance is one of the central concepts of modern semiconductor optics. In these cases one deals with an optically driven semiconductor probed by a weak light. A high-intensity pulse, which resonates with excitons and gives rise to the optical Stark effect, tends to weaken the exciton-photon interaction, and thereby modifies the propagation and absorption of the coupled polariton modes.
A disadvantage of the use of the conventional (optical or electrical) Stark effect in real devices is the requirement that real electronic excitations be created in the solid, with the attendant electrical losses and concomitant reduction in speed of operation.
If a sound wave is present in a solid, the density perturbations produced by rarefaction and compression as the sound wave passes lead to small modulations of the dielectric constant, and therefore the refractive index of the material. These spatial modulations can be used to diffract light, and this is the basis of the science of acousto-optics. Acousto-optics is a well-established discipline, but all of the current applications rely purely on the spatial modulation of the refractive index and deal with a small, non-resonant acousto-optical susceptibility.